The present invention relates generally to the field of medical imaging, and more specifically to the field of imaging geometry calibration and reconstruction. In particular, the present invention relates to three-dimensional imaging in a non-calibrated geometry.
Volumetric imaging devices are widely used in both medical and non-medical imaging fields. For example, various modalities serve to generate image data based upon specific physical properties of materials and their interaction with imaging systems. Such modalities include computed tomography (CT) systems, X-ray systems, magnetic resonance imaging (MRI) systems, positron emission tomography (PET) systems, ultrasound systems, and so forth. These systems are generally designed to collect image data in response to various radiation, stimuli, or signal sources emitted by or transmitted through a subject. The signals can then be filtered and manipulated to form a dataset from which a reconstructed image may be generated. It should be noted that while reference is made throughout the present discussion to modalities employed in the medical imaging field, these same and other modalities may be employed in a wide range of other fields, including baggage processing, human and non-human screening, industrial quality control, manufacturing monitoring, seismography, meteorology, and so forth.
A simple X-ray imaging technique may involve generating X-rays using an X-ray tube or other X-ray source and directing the X-rays through an imaging volume in which the part of the patient to be imaged is located. As the X-rays pass through the patient, the X-rays are attenuated based on the composition of the tissue they pass through. The attenuated X-rays then impact a detector that converts the X-rays into signals that can be processed to generate an image of the part of the patient through which the X-rays passed based on the attenuation of the X-rays. Three-dimensional information may be obtained by acquiring additional images at different viewing angles relative to the imaging volume. The angularly displaced images acquired in  this manner may then be reconstructed to produce a three-dimensional representation of the imaging volume including internal structures, which may be displayed on a monitor, printed to a printer, or reproduced on film. A technologist or clinician may then review the three-dimensional representation, such as to detect clinically significant irregularities or abnormalities or to assess the three-dimensional landscape prior to an invasive or non-invasive medical procedure. The reconstructed volumetric dataset may also be used for further processing, such as, for example, computer assisted detection and/or diagnosis (CAD).
In order to generate an accurately reconstructed three-dimensional image from the data produced by the various modalities discussed above, it is important to know the imaging geometry accurately. That is, the positions of the source, detector, and imaged volume relative to one another must be accurately known in order to determine how to properly reconstruct and/or combine the data to generate a true representation of the imaged volume. For example, in some imaging modalities, such as C-arm systems and tomosynthesis, the imaging geometry may change with each new data acquisition.
Currently, an imaging system must be calibrated in order to accurately reconstruct three-dimensional images from data obtained on the system. This calibration may include measuring the distances and angles between the elements of the system or imaging using one or more calibration phantoms. Measuring system components can be time-consuming and is not always accurate enough to produce good results. Calibration using appropriate phantoms may be more accurate than simply measuring the imaging system, however this technique may be limited to systems in which the imaging geometry is consistent and repeatable. In some cases, the system simply cannot be calibrated before a subject is imaged. For example, the position or trajectory of the imaging system may be determined in real-time as a function of patient anatomy or other factors, or the mechanical or motion control tolerances of the system may not be sufficiently accurate.
Therefore, it would be advantageous to have a method for determining the imaging geometry and reconstructing a volumetric image in a three-dimensional  imaging system where prior calibration of the imaging system is not possible or is not practical.